1. Field of the Invention
This invention relates to dielectric compositions for use, for example, in ceramic capacitors in particular, but not exclusively, multilayer ceramic capacitors.
2. Description of the Prior Art
A multilayer ceramic capacitor basically comprises a stack consisting of a plurality of dielectric members formed of a ceramic material, with electrodes positioned between the members. The electrodes may be screen-printed onto the ceramic material, in the unfired state thereof, using conductive inks. A stack of screen-printed dielectric members is assembled, pressed together, cut into individual components, if appropriate, and fired until sintering occurs, in order to ensure non-porosity.
With the originally employed dielectrics the capacitors had to be fired at temperatures of the order of 1200.degree.-1400.degree. C., which meant that the internal electrodes had to be of a suitable material to withstand such temperatures and that, therefore, expensive noble metals, such as platinum or palladium, had to be used. However, by suitable choice of the dielectric it is possible to reduce the firing temperature thus enabling the use of internal electrodes with a high silver content (50-100% silver), which reduces the cost of materials and manufacture. A dielectric composition which can be fired at a temperature between 950.degree. and 1100.degree. C. and can thus be used with high silver content internal electrodes is, disclosed in our U.S. Pat. No. 4,482,935 issued Nov. 13, 1984. The compositions disclosed therein comprise non-stoichiometric lead magnesium niobate (PbMg.sub.1/2 Nb.sub.1/2 O.sub.3) with one or more of the following, namely lead titanate, lead stannate, lead zirconate. Some of the compositions have dielectric constants in the range 7500-10000 which makes them particularly suitable for multilayer ceramic capacitors. The originally employed ceramics (U.S. coding Z5U) were not compatible with high silver content electrodes and usually had dielectric constants lower than 7500-10,000.
The electronics industry generally requires smaller components and smaller and cheaper capacitors can be obtained by producing dielectrics which are compatible with high silver content electrodes and have even higher dielectric constants than these with the 7500-10000 range mentioned above. One such composition is disclosed in our U.S. Pat. No. 4,525,768 issued June 25, 1985 and comprises non-stoichiometric lead magnesium niobate, non-stoichiometric lead iron niobate and one or more oxide additives, which may be chosen from silica, manganese dioxide, ceric oxide, lanthanum oxide, zinc oxide, alumina, tungsten oxide, nickel oxide, cobalt oxide and cuprous oxide. Additionally lead titanate may be included. If, for example, three or more oxide additives are chosen from the first eight of the ten mentioned above, compositions having firing temperatures between 900.degree. and 1075.degree. C. may be obtained, the dielectric constants after firing being in the range 10,600 to 16,800 making them particularly suitable for small multilayer ceramic capacitors.
In our further U.S. Pat. No. 4,536,821 issued Aug. 20, 1985 there is disclosed a dielectric composition based on non-stoichiometric lead magnesium niobate together with lead zinc niobate. This dielectric composition may also include one or more simple oxide additives chosen from silica, manganese dioxide, zinc oxide, nickel oxide, alumina, ceric oxide, lanthanum oxide, tungsten oxide, gallium oxide, titanium dioxide and lead oxide. One or more of the following may also be added to the basic composition, bismuth stannate, bismuth titanate, lead stannate, lead zirconate and lead titanate with or without a simple oxide additive. Such compositions fire at temperatures between 980.degree. and 1075.degree. C. and have dielectric constants at 25.degree. C. in the range 9000 to 16,300 with Z5U temperature dependence characteristics and low tan .delta. (%) (dielectric loss) at 25.degree. C.
It should be noted that the lead magnesium niobate employed in the above dielectric compositions is non-stoichiometric and is not the conventional stoichiometric PbMg.sub.1/3 Nb.sub.2/3 O.sub.3. In some of the specifications referred to above the expression PbMg.sub.1/2 Nb.sub.1/2 O.sub.3 is employed to distinguish from the conventional PbMg.sub.1/3 Nb.sub.2/3 O.sub.3. The material employed for the results quoted in the above specifications is in fact PbMg.sub.0.443 Nb.sub.0.5001 O.sub.3 which approximates to PbMg.sub.1/2 Nb.sub.1/2 O.sub.3. Preferably the magnesium was in the range 0.35 to 0.5 and the niobium was in the range 0.4 to 0.6 and thus the lead magnesium niobate was non-stoichiometric. The expression lead zinc niobate is conventionally understood to mean PbZn.sub.1/3 Nb.sub.2/3 O.sub.3, however non-stoichiometric versions are also possible and that used in U.S. Pat. No. 4,536,821 was defined as PbZn.sub.0.3 to 0.5 Nb.sub.0.6 to 0.7 O.sub.3.
In our co-pending U.S. patent application Ser. No. 706,790 now U.S. Pat. No. 4,625,258 issued Nov. 25, 1984 there is disclosed a ternary system dielectric composition comprising a mixture of non-stoichiometric lead magnesium niobate, non-stoichiometric lead zinc niobate and non-stoichiometric lead iron niobate, with or without lead oxide PbO as an addition at up to the 10wt % level, for example 5wt %. The lead oxide appeared to widen the firing range and gave dielectrics with useful properties at low firing temperatures, for example 950.degree. C. Some of the compositions disclosed in this further application had temperature coefficients of capacitance in the Z5U range, namely between 10.degree. C. and 85.degree. C. the capacitance variation remains within the band +22% to -56% of the 25.degree. C. value. Others of the compositions fell in the Y5V range, namely between -30.degree. C. and 85.degree. C., the capacitance variation remains within the band +22% to -82% of the 25.degree. C. value. Firing temperatures down to 900.degree. C. are possible with some compositions.